1. Field of the Invention
The invention generally relates to a lens, and more particularly, to a fixed-focus lens.
2. Description of Related Art
A fixed-focus lens is generally used in an optical system. The fixed-focus lens is widely applied therein, such as a monitoring camera, an automotive camera, a rear projection TV, a projector, etc. Along with development of optoelectronic technology, projectors having high quality have become more and more popular in recent years. In order to reduce the distance from the projector to the screen where images are projected and obtain great projected images, a fixed-focus lens having wide field of view is usually adopted therein. However, the images projected thereby may have aberration, such as distortion, field curvature, and astigmatism. Accordingly, in related arts, in order to improve aberration of the projected images, a plurality of aspheric lenses are utilized, or the number of lenses used therein and an overall length thereof are increased.
Moreover, in order to reduce the overall length thereof while improving aberration, a reflector may be added into the fixed-focus lens. For example, in U.S. Pat. No. 7,009,765, sixteen lenses and one reflector are used to reduce the overall length thereof while improving aberration. Furthermore, in US Patent Application Publication No. 2004/0233394, eighteen lenses and two reflectors are used to achieve the above-described objective. Moreover, a plurality of reflectors are used to reduce the distance form the projector to the screen where images are projected in U.S. Pat. No. 5,477,394; 6,994,442; 6,989,936; 6,690,517; or 6,984,044. However, it may enhance difficulty of manufacturing and fabricating the lens to increase the number of lenses used therein and to add the plurality of reflectors thereinto for reducing the overall length thereof while improving aberration, and cost of the projector is increased.